The present invention relates to a method for colouring the surface of tin and tin-containing articles, such as articles of pewter and other alloys of tin.
The colouring of metals is well known, with techniques for the colouring of such metals as copper having been well known for many years. However, comparatively little is known regarding the colouring of tin and tin-containing alloys. In part this is due to the generally high level of inertness exhibited by tin.
Pewter is an alloy of tin, containing from 85 to 99% by weight of tin and from 1 to 15% by weight of copper to provide the alloy with a degree of hardness. Traditionally, lower grades of pewter have contained lead. However, modern trends are to replace the lead with antimony or bismuth. It is known to colour the surface of pewter articles, in particular to darken the article to replicate the dark grey patina of old lead pewter. Specific techniques for darkening the surface of pewter articles are disclosed in Publication No. 566 ‘Working with pewter’, published by the International Tin Research Institute.
U.S. Pat. No. 3,930,898 discloses a process for imparting a dark patina to the surface of an article formed from a low melting metal alloy, including alloys of tin such as pewter. The method comprises exposing the surface of the article to an acid solution of a copper salt, in particular copper sulphate, copper chloride or copper nitrate.
U.S. Pat. No. 6,013,328 concerns a electroless method of antiquing a pewter component, the method comprising contacting the component with an oxidizing solution containing copper chloride, selenious acid, hydrochloric acid and sodium tetradecyl sulphate.
Other techniques for colouring tin and pewter are referred to in the art. Harris, C. ‘Colouring Miscellaneous Metals: silver-tin-gold-zinc-cadmium-nickel’, Metal Industry, January 1951, pages 71 to 73, describes various electrolytic colouring processes for tin and pewter, similar to those applied to aluminium. Further, various lacquering techniques have been practiced for a number of centuries.
EP 1 336 669 discloses a process for providing a tortoise-shell chromatic effect on a metallic substrate. The process involves heating the metallic substrate to a temperature of from 100° C. up to 140° C. in the presence of an aqueous solution comprising sodium thiosulfate, copper acetate and potassium acid tartarate. The process is particular aimed at the colouring of nickel.
SU 19853912493 concerns the removal of copper from tin alloys by treating with sodium thiosulphate at temperatures from 250 to 500° C.
US 2004/0104122 is directed to a process for the passivating of tin, zinc and steel surfaces by applying a coating of a protective composition. In the case of tin, the coating is of a composition comprising proteins, amino acids and amine alcohols.
JP 2002191421 discloses a process for colouring a gold alloy ornament, in which the surface of the gold alloy is coated in a very thin film of tin and then treated with an aqueous solution of acetic acid, copper sulfate and table salt; ammonium chloride and dilute sulfuric acid; copper sulfate, potassium sulfate, sodium salt and caustic soda; copper sulfate, green rust and alum and a mixed aqueous solution of sodium thiosulfate and lead acetate; copper acetate, copper nitrate, ammonium chloride, alum and mercury (II) chloride, copper (I) carbonate and arsenious acid with a mixed aqueous solution of copper sulfate, alum, mercury (II) chloride, copper nitrate and ammonium chloride. The treatment forms verdigris on the surface of the gold ally ornament.
U.S. Pat. No. 3,767,475 discloses a method of plating metallic articles with tin and treating the tin plating to render it oxidizable. Thereafter, the tin layer is oxidized.
While certain techniques for colouring tin are known, the practice of colouring tin-containing articles is by no means as well developed as with many other metals. As a result, there is a need for a simple method for colouring tin and tin-containing articles, such as pewter articles. It would be an advantage if the method could be applied with only simple modifications to provide a range of different colourations on the surface of the article.